The present invention is concerned with the measuring instruments and relates more particularly to a process and a device for measuring the content of uncondensable substances present in a gaseous mixture, especially steam considered as a heat-exchange fluid.
It will be recalled that the water intended for feeding boilers and coming from the distribution network generally undergoes two operations before it enters the boiler, namely passage through an ion-exchanger column for the purpose of demineralization, and thermal degassing; in spite of this degassing, a residual content of air remains in the water and appears again in the steam in the form of uncondensable substances. Another reason for the presence of air in the steam is that the pipelines and the vaporization or exchange vessel are bled insufficiently at the outset.
When steam is used as a heat-exchange fluid in an exchanger, the overall result of the presence of uncondensable substances in the steam is, on the one hand, a reduction in the heat flux between the steam and the exchange wall and, on the other hand, the occurrence of zones which are superheated in relation to other zones on the exchange wall.
These disadvantages attributable to the presence of uncondensable substances are usually overcome by means of periodic or continuous bleeding, that is to say the rejection of steam outside the exchanger; this bleeding is generally carried out systematically from time to time without any specific criterion, that is to say irrespective of the actual content of uncondensable substances in the steam; the result of this procedure is that either bleeding is insufficient and then gives rise to the above-mentioned disadvantages, or bleeding is too frequent and results in corresponding energy losses. Of course, the logical method is to bleed the system as a function of the content of uncondensable substances present in the steam, and this implies an evaluation of the content of the latter.
Two methods of measuring the content of uncondensable substances are known, both usually requiring a weighing operation; according to a first method, a representative portion of the gaseous mixture is extracted and condensed in a vessel containing cold water, underneath a measuring cylinder turned upside down in the form of a gasholder; after a certain time, the volume of gas collected is read off, the increase in the weight of water in the vessel attributable to the condensate is measured and the content of uncondensable substances is deduced from this; such a method is impractical, particularly because it is intermittent. A second method involves separately measuring, on the one hand, the instantaneous flowrate of the condensates by weighing and, on the other hand, the instantaneous flowrate of the uncondensable gases, for example by measuring the pressure in a vessel with gauged leakage, the ratio of these two flowrates giving the content of uncondensable substances; however, where low flowrates are concerned, particularly regarding gases, the latter method of measurement is especially inaccurate, above all for low contents of uncondensable substances.